Make use of Win32 functions conditional on _WIN32, not _MSC_VER.

Using _MSC_VER here means that the choice of signal() versus
SetConsoleCtrlHandler depends on the compiler being used, rather
than the OS being targeted. When built with MinGW rather than MSVC,
this happens to work because MinGW's signal emulation is used, but
that emulation is quite limited.

Instead, be consistent and use the Win32 API when building for that
platform, regardless of compiler.

Note that if building for Cygwin, _WIN32 is not defined and POSIX
APIs are used.

host/hackrf-tools/src/hackrf_transfer.c

@@ -609,7 +609,7 @@ static void usage()
 
 static hackrf_device* device = NULL;
 
-#ifdef _MSC_VER
+#ifdef _WIN32
 BOOL WINAPI sighandler(int signum)
 {
 	if (CTRL_C_EVENT == signum) {
@@ -1041,7 +1041,7 @@ int main(int argc, char** argv)
 		fwrite(&wave_file_hdr, 1, sizeof(t_wav_file_hdr), file);
 	}
 
-#ifdef _MSC_VER
+#ifdef _WIN32
 	SetConsoleCtrlHandler((PHANDLER_ROUTINE) sighandler, TRUE);
 #else
 	signal(SIGINT, &sigint_callback_handler);